An engine may be operated at higher engine speeds and loads from time to time. Operating the engine at higher loads may increase engine exhaust gas temperatures, and operating the engine at higher speeds may increase the exhaust gas mass flow rate. Consequently, the amount of thermal energy supplied to an engine exhaust system may increase at higher engine speeds and loads. For example, a temperature of an exhaust gas after treatment device, such as a catalyst, may increase as engine speed and load increase. Further, the exhaust gas after treatment device temperature may increase further when hydrocarbons discharged from the engine oxidize within the after treatment device.
One way to reduce engine exhaust gas temperature and catalyst temperature is to enrich the engine air-fuel ratio to a ratio that is rich of stoichiometry. However, enriching the engine air-fuel ratio may increase hydrocarbon and carbon monoxide emissions from the engine. Further, engine fuel consumption may increase beyond what a driver may expect given the driver's torque input request.
The inventor herein has recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: advancing spark timing from a borderline (BDL) spark timing toward minimum spark advance for best torque (MBT) and enriching an engine air-fuel ratio in response to a catalyst temperature greater than a threshold temperature.
By advancing spark timing toward MBT timing in response to catalyst temperature, it may be possible to provide the technical result of reducing engine thermal output without increasing engine emissions and fuel consumption. For example, engine torque may be limited or constrained to a torque where MBT spark timing occurs without encountering engine knocking in response to catalyst temperature. Thus, an engine that was operating at borderline spark timing before a catalyst temperature threshold was exceeded, may be operated with advanced engine spark timing and at a lower torque in response to catalyst temperature greater than a threshold temperature. In this way, catalyst temperature may be limited or reduced without increasing engine fuel consumption.
The present description may provide several advantages. Specifically, the approach may reduce engine fuel consumption. Additionally, the approach may reduce the possibility of catalyst degradation. Further still, the approach may improve vehicle drivability at higher engine loads.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.